Until a few years ago, polychlorinated biphenyls (PCB's) were widely used as dielectric fluids in transformers and capacitors. However, when it was discovered that PCB's persisted in the environment and had deleterious effects on wildlife and possibly people, government regulations began to prohibit their use. As PCB's were phased out, it was necessary to find other dielectric fluids of comparable electrical, chemical, and physical properties.
One of the most important electrical properties for a dielectric fluid is a high dielectric strength, particularly a high partial discharge onset and offset voltage. While many of the fluids that were substituted for PCB's were comparable in other properties, in these two properties they were often deficient.